This invention relates to varactors, and more particularly, to metal-oxide-semiconductor varactors with multipart gate structures.
A varactor is a controllable capacitor. Varactors are widely used in high frequency integrated circuits. For example, varactors are commonly used as voltage-controlled tuning components in analog circuits such as filters and voltage-controlled oscillators. Voltage-controlled oscillators are used in circuits such as phase-locked loops. Phase-locked loops, in turn, are often used in circuits such as clock and data recovery circuits.
Successful operation of these circuits depends on accurate and predictable varactor performance. High performance varactors are characterized by high quality factors (“Q”), a wide and gradual voltage-controlled capacitance tuning range, and good linearity.
Shortcomings in these performance attributes can adversely affect circuit performance. For example, non-linear and abrupt varactor tuning characteristics can adversely affect noise performance and circuit stability.
Varactors have been developed that are based on pn junction capacitance (“junction varactors”) and modified metal-oxide-semiconductor (MOS) transistor structures (“MOS varactors”). MOS varactors that are operated in accumulation mode are sometimes referred to as accumulation mode metal-oxide-semiconductor (A-MOS) varactors. MOS varactors that are operated in inversion mode are sometimes referred to as inversion mode metal-oxide-semiconductor (I-MOS) varactors.
Junction varactors exhibit gradual changes of capacitance with change in tuning voltage. This gradual CV slope characteristic is generally beneficial for circuit performance. Nevertheless, junction varactors exhibit poor quality factors Q and poor tuning ranges.
Because of these issues, many modern circuit designs use MOS varactor structures. Both A-MOS and I-MOS varactors exhibit satisfactory tuning ranges and quality factors. The quality factor of A-MOS devices tends to be superior to that of I-MOS devices. Moreover, I-MOS devices tend to exhibit particularly steep CV slopes. A-MOS devices are therefore often preferred over I-MOS devices.
A-MOS varactors tend to exhibit superior quality factors and tuning ranges to those available from junction varactors. Nevertheless, A-MOS varactors exhibit steeper CV slopes than junction varactors. The relatively steep slope of A-MOS varactors has made conventional A-MOS varactors unappealing in some design contexts. As a result, analog circuit designers sometimes prefer to use junction varactors, despite their poorer quality factor and tuning range characteristics. These constraints sometimes force designers to make undesirable design compromises.
It would therefore be desirable to be able to provide improved varactors for integrated circuits.